A lexical decision task, or LTD, is a type of psychological trial in which an individual is asked to determine whether or not a given sensory input corresponds to a word. The sensory information may be either visual, in the form of images or groups of letters, or auditory. The lexical decision task is particularly important to subfields of psychology known as semantic memory and lexical access. Semantic memory refers to the recollection of facts, meanings, and concepts that are not specifically linked to some historical point in an individual's life memory. Lexical access specifically refers to the process in which connections between stimuli and meaning are made, such as when a sound is identified as a word.

There are many different variables that researchers can alter in a lexical decision task in order to allow them to draw inferences about which conditions affect lexical access in which ways. How common or rare a word is, for instance, can have a significant impact on how fast, on average, individuals are able to identify a word. Researchers also try to get a broad sample of different test subjects to participate in a given task. An individual with a strong educational background, for instance, is likely to perform better than someone with very little schooling or with limited literacy.

Psychological trials of this sort are usually scored based on how long it takes a test taker to identify a sound or image as a word or non-word. To do so, the test taker is generally asked to press a button on a computer or to do some other decisive, quick action. In many cases, the entire lexical decision task is conducted on a keyboard. This eliminates the risk that the direct presence of the researcher could have any kind of effect on the results.

Repetition and word association are two phenomena that have been demonstrated to have particular significance in a lexical decision task experiment. A word identified as a word at one point in the test will generally be recognized as a word more quickly later in the test. Additionally, a word that is closely associated with a previous word will often be recognized more quickly than a word without prior associations. "Table," for instance, is likely to be recognized more quickly in a lexical decision task if it is preceded by "chair" than if it is preceded by "walrus."

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